96 research outputs found
Vortex dynamics in superconducting channels with periodic constrictions
Vortices confined to superconducting easy flow channels with periodic
constrictions exhibit reversible oscillations in the critical current at which
vortices begin moving as the external magnetic field is varied. This
commensurability scales with the channel shape and arrangement, although
screening effects play an important role. For large magnetic fields, some of
the vortices become pinned outside of the channels, leading to magnetic
hysteresis in the critical current. Some channel configurations also exhibit a
dynamical hysteresis in the flux-flow regime near the matching fields
Picovoltmeter for probing vortex dynamics in a single weak-pinning Corbino channel
We have developed a picovoltmeter using a Nb dc Superconducting QUantum
Interference Device (SQUID) for measuring the flux-flow voltage from a small
number of vortices moving through a submicron weak-pinning superconducting
channel. We have applied this picovoltmeter to measure the vortex response in a
single channel arranged in a circle on a Corbino disk geometry. The circular
channel allows the vortices to follow closed orbits without encountering any
sample edges, thus eliminating the influence of entry barriers.Comment: 4 pages, 3 figures, submitted to Review of Scientific Instrument
Depairing currents in superconducting films of Nb and amorphous MoGe
We report on measuring the depairing current J_{dp} in thin superconducting
films as a function of temperature. The main difficulties in such measurements
are that heating has to be avoided, either due to contacts, or to vortex flow.
The latter is almost unavoidable since the sample cross-section is usually
larger than the superconducting coherence length \xi_s and the magnetic field
penetration depth \lambda_s. On the other hand, vortex flow is helpful since it
homogenizes the distribution of the current across the sample. We used a pulsed
current method, which allows to overcome the difficulties caused by dissipation
and measured the depairing current in films of thin polycrystalline Nb (low
\lambda_s, low specific resistance \rho) and amorphous Mo_{0.7}Ge_{0.3} (high
\lambda_s, high \rho), structured in the shape of bridges of various width. The
experimental values of J_{dp} for different bridge dimensions are compared with
theoretical predictions by Kupriyanov and Lukichev for dirty limit
superconductors. For the smallest samples we find a very good agreement with
theory, over essentially the whole temperature interval below the
superconducting critical temperature.Comment: 5 pages, 6 figure
Depth dependent spin dynamics of canonical spin glass films: A low-energy muon spin rotation study
We have performed depth dependent muon spin rotation/relaxation studies of
the dynamics of single layer films of {\it Au}Fe and {\it Cu}Mn spin glasses as
a function of thickness and of its behavior as a function of distance from the
vacuum interface (5-70 nm). A significant reduction in the muon spin relaxation
rate as a function of temperature with respect to the bulk material is observed
when the muons are stopped near (5-10 nm) the surface of the sample. A similar
reduction is observed for the whole sample if the thickness is reduced to e.g.
20 nm and less. This reflects an increased impurity spin dynamics (incomplete
freezing) close to the surface although the freezing temperature is only
modestly affected by the dimensional reduction
Superconducting properties of Nb thin films deposited on porous silicon templates
Porous silicon, obtained by electrochemical etching, has been used as a
substrate for the growth of nanoperforated Nb thin films. The films, deposited
by UHV magnetron sputtering on the porous Si substrates, inherited their
structure made of holes of 5 or 10 nm diameter and of 10 to 40 nm spacing,
which provide an artificial pinning structure. The superconducting properties
were investigated by transport measurements performed in the presence of
magnetic field for different film thickness and substrates with different
interpore spacing. Perpendicular upper critical fields measurements present
peculiar features such as a change in the H_c2(T) curvature and oscillations in
the field dependence of the superconducting resistive transition width at H=1
Tesla. This field value is much higher than typical matching fields in
perforated superconductors, as a consequence of the small interpore distance.Comment: accepted for publication on Journal of Applied Physic
Proximity effects in the superconductor / heavy fermion bilayer system Nb / CeCu_6
We have investigated the proximity effect between a superconductor (Nb) and a
'Heavy Fermion' system (CeCu_6) by measuring critical temperatures and
parallel critical fields H_{c2}^{\parallel}(T) of Nb films with varying
thickness deposited on 75 nm thick films of CeCu_6, and comparing the results
with the behavior of similar films deposited on the normal metal Cu. For Nb on
CeCu_6 we find a strong decrease of T_c with decreasing Nb thickness and a
finite critical thickness of the order of 10 nm. Also, dimensional crossovers
in H_{c2}^{\parallel}(T) are completely absent, in strong contrast with Nb/Cu.
Analysis of the data by a proximity effect model based on the Takahashi-Tachiki
theory shows that the data can be explained by taking into account both the
high effective mass (or low electronic diffusion constant), {\it and} the large
density of states at the Fermi energy which characterize the Heavy Fermion
metal.Comment: 7 pages, 2 figure. Manuscript has been submitted to a refereed
journa
Charge carrier localization due to ferromagnetic clusters in concentrated AuFe alloys
Quantum Matter and Optic
Depairing currents in the superconductor/ferromagnet proximity system Nb/Fe
We have investigated the behaviour of the depairing current J_{dp} in
ferromagnet/superconductor/ferromagnet (F/S/F) trilayers as function of the
thickness d_s of the superconducting layers. Theoretically, J_{dp} depends on
the superconducting order parameter or the pair density function, which is not
homogeneous across the film due to the proximity effect. We use a proximity
effect model with two parameters (proximity strength and interface
transparency), which can also describe the dependence of the superconducting
transition temperature T_c on d_s. We compare the computations with the
experimentally determined zero-field critical current J_{c0} of small strips
(typically 5~ \mu m wide) of Fe/Nb/Fe trilayers with varying thickness d_{Nb}
of the Nb layer. Near T_c the temperature dependence J_{c0}(T) is in good
agreement with the expected behaviour, which allows extrapolation to T = 0.
Both the absolute values of J_{c0}(0) and the dependence on d_{Nb} agree with
the expectations for the depairing current. We conclude that J_{dp} is
correctly determined, notwithstanding the fact that the strip width is larger
than both the superconducting penetration depth and the superconducting
coherence length, and that J_{dp}(d_s) is correctly described by the model.Comment: 10 pages, 5 figures, submitted to PR
- …